Feature Request: optionally use sidecar-based heartbeat for ReplicationLagSeconds in health stream

[timvaillancourt]

This sub-task discusses using the age of the heartbeat timestamp in the sidecar-based heartbeat (_vt.heartbeat) as the ReplicationLagSeconds in health check stream updates from vttablet

Using this value is more accurate because it:

• Captures lag when the disk has stalled
• Captures the lag of the IO + SQL thread executing binlog changes
• Captures the lag introduced by the network between the replica and primary